This invention relates to apparatus for expanding and flaring tubing, for example, copper heat exchanger tubes. The invention is more particularly directed to apparatus for belling the ends of heat exchanger tubes and expanding the tubes in a plate-fin heat exchanger.
Plate-fin heat exchangers are often employed in air conditioning systems and refrigeration systems, e.g., for trucks, seagoing vessels, and railroads. These plate-fin units are typically formed by lacing so-called hairpin tubes or U-tubes into aligned holes in a stack of fin plates and tube sheets, with the U-bend sections extending out one side of one of the tube sheets. The open ends of the tubes extend out the other tube sheet. The walls of the tube, which is typically copper, are then expanded radially into contact with the metal of the fin collars and the tube sheets. This establishes good thermal contact and mechanical support. The hairpin tube ends are belled, either before or after tube expansion, the return bends are soldered or brazed into the belled ends to close the flow circuit of the unit.
In compression expansion, the hairpin tubes are supported from the U-bend side, and the belling typically takes place after the expansion. This technique can result in uncertainty in establishing an offset or standoff distance between the tube belled ends and the tube sheet. Because of varying amounts of tube shrinkage during expansion, the bells are often incompletely formed, requiring significant reworking in some cases, and resulting in scrapping of the fin pack in other cases. A compression-expansion technique is described in U.S. Pat. No. 4,228,573.
Tension expansion involves gripping a length of the open end of the tubes, generally a three-inch length, and belling the hairpin tubes prior to expansion, and then supporting the tubes by the tube sheet or their belled ends while expander rods are driven into the two legs of each hairpin tube. The tubes can be belled directly against the associated tube sheet so that the tube sheet supports the hairpin tubes during expansion, or else the bells can be formed at an established standoff distance above the tube sheet. In the latter case, the belled ends can be supported in a clamping jaw or similar device during expansion. One technique for belling and expanding hairpin tubes in a finpack heat exchanger is described in U.S. Pat. No. 4,584,765.
To date there has been no equipment or apparatus proposed which permits the belling of short lengths of the hairpin tube, generally less than one inch in length, to be carried out simply and reliably, or which permits the belled ends of the heat exchanger tubes to be offset reliably at an established, finite distance from the associated tube sheet and eliminates the scratching of the bell that typically can occur if a long, serrated clamp is used, and which weakens the tube.